1. Technical Field of the Invention
The present invention relates in general to the telecommunications field and, in particular, to echo cancellation in telephony systems.
2. Description of Related Art
"Echo" is a phenomenon that can occur in a telephony system whenever a portion of transmitted speech signal energy is reflected back to a sender. These reflections are caused by impedance mismatches in analog portions of the telephony network. There can be many different sources of echo, such as, for example, a hybrid circuit that converts a 4-wire line to a 2-wire line in a Public Switched Telephone Network (PSTN) subscriber interface, or acoustical cross-talk in a mobile radiotelephone. The presence of echo along with a substantial delay (e.g., physical distance or processing delay) can severely degrade the quality of the speech signals being processed.
An echo canceller is a device that is commonly used in telephony systems to suppress or remove echoes in long distance traffic. For example, in cellular Public Land Mobile Networks (PLMNs), echo cancellers are used in mobile services switching centers (MSCs) to suppress or remove echoes in speech traffic. Echo cancellers are also used in mobile radiotelephones and "handsfree" telephone equipment to compensate for acoustical echoes. A general description of an existing echo cancellation technique can be found in the paper entitled: "A Double Talk Detector Based on Coherence" by Gansler et al, Signal Processing Group, Dept. of Elec. Eng. and Comp. Science, Lund University, Sweden.
In principle, a digital mobile radiotelephone handset should not generate echoes, because the connection used comprises 4-wires down to the handset. In practice, however, many such mobile phones generate echoes that originate from acoustical or mechanical cross-talk in the handset. This type of echo is annoying to users, especially if the system operator has elected to raise the downlink signal levels. Raising the downlink signal levels has become an accepted practice, since many customers have complained about the low output levels from the mobile handsets, ' speakers.
Existing telephony systems that provide long distance traffic and MSCs in Public Land Mobile Networks (PLMNs) employ echo cancellers to control echoes generated in the PSTN side of the connection. For example, FIG. 1 is a simplified schematic block diagram of a conventional echo canceller (10) used in long distance traffic systems and MSCs. The main component of such an echo canceller is an adaptive finite-impulse-response (FIR) filter 12. Under the control of an adaptation algorithm (e.g., executing in software), filter 12 models the impulse response of the echo path. A non-linear processor (NLP) 14 is used to remove residual echo that may remain after linear processing of the input signal. A double talk detector (DTD) 16 is used to control and inhibit the adaptation process, when the echo signal to "near end" signal ratio is of such a value that no additional improvement in the echo path estimation can be obtained by further adaptation of filter 12. The block denoted by 18 represents the echo source in the telephony system which generates the "desired" signal, y(t), as a function of the "far end" signal, x(t), and the "near end" signal, v(t). A comfort noise generator (CNG) 20 is used to generate a noise signal which is essentially similar to the background noise at the "near end". This noise signal is inserted into the connection while the NLP 14 is active. It is generally accepted that an echo canceller should be switched off on those connections that carry high bit-rate data traffic with "V-series" modems, since these modems include their own echo cancellers. For this purpose, network echo cancellers typically include a tone disabler (TD) 22, which detects the modem's answering tone (e.g., 2100 Hz tone) and disables some or all of the echo canceller's functions if answering tones with certain predetermined characteristics are received.
There are a number of differences between the characteristics of the echo signals that originate in PSTNs and those that originate in digital mobile phones. For example, the echo path from a PSTN is quasi-linear and, therefore, can be readily modeled by a linear filter. A number of existing solutions, which are based on adaptive filtering techniques, can be used successfully to cancel these types of echoes. The length of the echo path "seen" from an echo canceller in such a network can be up to 64 ms, which implies the use of an adaptive filter of up to 512 taps to model the echo path. The computational resources required to execute the adaptive algorithm for that long a filter consume a large portion of the capacity of the digital signal processor (DSP) which makes up the echo canceller. Furthermore, the echo return loss (ERL) from a PSTN depends on the balance circuitry used in the network. As a general rule, the ERL (measured in dB) can be considered as a random variable selected from a Gaussian distribution, with a mean of 13.6 dB and a standard deviation of 2.8 dB for a segregated loop balancing scheme.
On the other hand, the echo path for a digital mobile phone is non-linear and time-varying, due to the use of two speech coder/decoder (codec) pairs and radio interfaces in the transmission path. Additionally, the level of the echo in a digital mobile phone is much lower than that from a PSTN. For example, the specification for the digital cellular Global System for Mobile Communications (GSM) requires an ERL of 46 dB (for the mobile phones) measured for pure tones of level 0 dBm in the 300-3400 Hz band. However, the ERL can be lower if signals other than pure tones are used for the measurements, but suppression levels of about 40 dB can still be expected. In other words, the quantization noise appears to be a considerable source for the echo path non-linearity. In fact, the ERL from a digital mobile phone is comparable to the ERL that can be obtained by the linear filter portion of a conventional PSTN echo canceller. For the above-described reasons, it is unlikely that the echo from a digital mobile phone would be cancelled by more than a few dB by a linear filter.
As such, the existing echo cancellers are designed to cancel echo that originates from only one side of the switching connection. Consequently, it follows that when echoes originating on both sides of the switching connection are to be cancelled, then two echo cancellers per connection are used.
In an exemplary configuration, a plurality of echo cancellers are integrated into a digital switching system. For example, a plurality of echo cancellers manufactured by Ericsson Radio Systems AB have been integrated into an Ericsson AXE 10 digital switching system. These echo cancellers form a part of the AXE 10 Trunk and Signalling Subsystem (TSS) and are directly connected to the group switch in a pool configuration (referred to as "ECP" or echo cancellers in a pool). The technique used to operate these echo cancellers in a network has been to connect a device to each trunk. In other words, when echo cancellation is needed, the AXE 10 selects one of the echo cancellers from the pool and routes the connection through the selected echo canceller. In this way, the AXE 10 ECP configuration can concentrate the traffic and thus reduce the total number of echo cancellers used, in comparison with the earlier direct connection trunk configurations used.
A problem arises if echoes from the two sides of the switching connection are controlled by separate echo cancellers. FIG. 2 is a diagram of an exemplary system (50) in which two echo cancellers 52 and 54 (e.g., two echo cancellers 10 shown in FIG. 1) are being used to control echoes originating on both sides of the connection in an MSC 56. Obviously, as illustrated by FIG. 2, double the usual number of echo cancellers are needed to control echoes from both sides of the connection, if separate devices are used. Furthermore, if the echo cancellers used are connected in a pool configuration (e.g., an ECP 101 or ECP 303 manufactured by Ericsson Radio Systems AB), the increase in the number of echo cancellers involved results in a proportional increase in the number of group switch multiple input/output positions required. Additionally, some other functions, such as the 2100 Hz tone detectors have to be included in both of the echo cancellers used.